In a typical Optical Sub-Assembly (OSA) a variety of component types, e.g. lasers, photodiodes, are attached to a single platform. The components are typically formed on semiconductor, usually III-V semiconductor, substrates and the platform is also usually a III-V semiconductor so that the thermo-mechanical characteristics of the platform matches those of the component substrates. The component substrates are attached to the platform substrate using solder bumps. The attachment technique should produce a reliable and cost-effective bond. Gold-tin solder is especially well suited for this application, and is the preferred means of attachment.
Several options are available for the solder preparation and solder bump bonding operations. In most gold-tin solder processes, the gold and tin are frequently deposited as separate layers. The preferred approach is to first deposit a gold-tin-gold stack. The gold-tin eutectic forms during reflow of the solder.
Electroplating techniques have been proposed for depositing the gold and tin layers on the substrates. This approach has the advantage of being a low cost, relatively well understood, process, capable of high volumes. However, there are several disadvantages in this approach, e.g., poor purity control of plated materials, high toxicity of the electrolytes, relatively large footprint of electroplating equipment.